Electroluminescent displays (ELDs) are light emissive electronic display devices. ELDs are categorized into inorganic electroluminescent devices (inorganic EL devices) and organic electroluminescent devices (organic EL devices).
Inorganic EL devices, which have been used as planar light source, require high AC voltage to be driven.
An organic EL device usually has a luminous layer containing a luminescent compound disposed between a cathode and an anode. In the organic EL device, electrons and holes are injected into a luminous layer to recombine therein, which generates excitons. The excitons are deactivated while emitting light (fluorescence or phosphorescence). The organic EL device emits light in such a manner. Organic EL devices can emit light at a low voltage in the range of several volts to several tens of volts. Organic EL devices also provide a wide viewing angle and high visibility due to their self-luminescent characteristics. Organic EL devices are also attracting attention from the viewpoints of space saving and portability because they are full solid state thin-film device.
Another great feature of organic EL devices is being a surface light source, unlike major light sources conventionally put into practical use, for example, light emitting diodes and cold cathode tubes. Applications of organic EL devices that effectively exploit such a property include light sources for lighting and backlights of various displays. Organic EL devices are particularly suitable for backlights of full-color liquid crystal displays that have been increasingly demanded in recent years.
If an organic EL device is used as such a light source for lighting or display backlight, the organic EL device is normally used as a light source which emits white light or so-called incandescent light (hereinafter, “white” and “incandescent” are collectively referred to as “white”). The organic EL device can also be used as a light source for lighting in the form of a panel which emits any color, such as blue, green, red, and intermediate colors thereof.
In a field of lighting, a light source for color-adjustable lighting has been demanded in recent years not only in the area of so-called design lighting but also in the area of general interior lighting. An organic EL device to be used as a light source for lighting also preferably has such a function.
Generally, an organic EL device has a basic layer structure composed of an anode, an organic luminous layer, and a cathode, and optionally has one or more layers, such as a hole injection layer, a hole transport layer, an electron transport layer, and electron injection layer.
An organic EL device with such a structure generally involves a tradeoff between luminance and driving lifetime. For solving the tradeoff, organic EL devices have been studied from various viewpoints, such as organic materials, the structure of the organic layer, and the driving mode of the devices.
From the viewpoint of the structure of the organic layer, as a technique for solving the tradeoff, a device structure has been proposed, which includes a laminate of several luminous units separated by one or more intermediate layers therebetween, each luminous unit including one or more luminous functional layers including a luminous layer composed of an organic material. Such a device has a prolonged lifetime while maintaining luminance, or can emit light with high luminance without a significant reduction in the driving lifetime.
Driving modes of a device with such a structure are classified into two major groups. One is to drive the device while the intermediate layer is being in an electrically floating state, and the other is to drive the device while the intermediate layer is connected to an external power source so as to serve as a so-called intermediate electrode and a voltage is directly applied to each light-emitting unit.
The latter mode can solve the tradeoff between the luminance and the device lifetime. In addition, an organic EL device that includes light-emitting units emitting light of mutually different colors can emit color-matched light through adjustment of brightness of individual light-emitting units.
PTLs 1 to 4 disclose exemplary structures of such an organic EL device having an intermediate electrode.
PTLs 1 and 2 each disclose a structures with an intermediate electrode composed of ITO (indium tin oxide) as a material.
ITO has high transparency and desirable properties for use as an intermediate electrode of an organic EL device. For formation of a film, sputtering is generally used. Unfortunately, such a film forming technique involves a risk of damage of an organic layer, resulting in unsatisfactory properties of the resulting organic EL device. In order to solve such a problem, several methods for preventing or reducing damage of the organic layer have been proposed, such as providing a metal thin film layer between an organic layer and an ITO electrode and applying a very low film formation rate in sputtering. However, such methods involve complex layer structure and production process and increased producing costs.
PTL 3 discloses an organic EL device including a charge generating layer containing an elemental metal or a metal compound that has a work function of 3 eV or lower and a compound with a work function of 4 eV or higher. The “compound with a work function of 4 eV or higher” is not a metal compound and thus is not so much electrically conductive as metal materials, that is, its conductivity is entirely inadequate to exhibit functions as an intermediate electrode. Furthermore, the charge generating layer disclosed in PTL 3 is not connected to an external power source.
PTL 4 discloses an organic EL device having an electrode layer composed of a mixture of a metal with a work function of 4.2 eV or lower, a metal with a work function of higher than 4.2 eV, and an organic compound, or an electrode layer composed of a laminate of sublayers which separately contain one of the aforementioned metals with an organic compound. Such an electrode layer is supposed to be used as a charge generating layer or a layer adjacent to an electrode with higher conductivity. Such a layer containing an organic compound has an entirely inadequate conductivity, which is lower than that of a layer composed of an elemental metal.